Ion beam image converter

ABSTRACT

The ion beam image converter comprises a secondary emission screen in the ion beam path so that, when ions impinge upon the secondary emission screen, secondary electrons are emitted. An accelerator screen is positioned downstream from the secondary emission screen to accelerate the secondary electrons. Deflecting means is positioned downstream of the accelerator screen to deflect the electron beam onto a path away from the ion beam path. A phosphor image screen is placed on the deflected secondary electron path to convert the electron beam to visible image. Electron beam deflection can be either electrostatic or magnetic.

United States Patent J amba et al.

[54] ION BEAM IMAGE CONVERTER [72] Inventors: Douglas M. Jamba, WoodlandHills; Robert M. Ennis, Jr., Malibu, both of 3,277,297 10/1966 Forresteret al. "250/495 Feb. 8, 1972 2,769,911 ll/l956 Warmoltz ..250/4l.9

Primary Examiner-William F. Lindquist Attorney-James K. Haskell andAllen A. Dicke, Jr.

[ ABSTRACT The ion beam image converter comprises a secondary emissionscreen in the ion beam path so that, when ions impinge upon thesecondary emission screen, secondary electrons are emitted. Anaccelerator screen is positioned downstream from the secondary emissionscreen to accelerate the secondary electrons. Deflecting means ispositioned downstream of the accelerator screen to deflect the electronbeam onto a path away from the ion beam path. A phosphor image screen isplaced on the deflected secondary electron path to convert the electronbeam to visible image. Electron beam deflection can be eitherelectrostatic or magnetic.

3 Claims, 2 Drawing Figures [ON BEAM IMAGE CONVERTER BACKGROUND Thisinvention is directed to an ion beam image converter, which is capableof providing a visible image corresponding to the original ion beam sothat gross beam characteristics, such as size and shape of the ion beamcross section, can be visualized, even for very low-current ion beams0.l pA).

One group of prior devices used for observing ion beams depended uponions striking material, such as quartz, or a phosphor, such that theyemit visible light. Direct ion bombardment of phosphors graduallydestroys the light-emitting properties, to thus limit the usefullifetime. Quartz detector plates are more resistant to bombardment, butthe visible light is diffused and reflected, distorting the images, tomake observation inaccurate and difficult. In another type of suchdevice, a metal screen is placed directly in front of the phosphor. Theion beam striking the metal screen generates secondary electrons whichare attracted through the screen and accelerated to the phosphor,producing an image of the ion beam. This device is usable to observeonly relatively low-energy ion beams, such as are obtained in ionmicroscope studies of cesium ion emission from porous tungsten. Thesuccess of this device depends upon maintaining the positive potentialon the phosphor screen higher than the beam energy, so that ions aredecelerated and are not allowed to strike the phosphor. This type ofdevice is limited to a maximum of about 20 k.e.v. beam energy. Abovethat energy level, the electrons striking the phosphor have a highenough energy to cause dangerous X- ray emission conditions. In allcases where higher energy ion beams are allowed to strike a surface,there is an inherent danger of generating X-rays of sufficient intensityto prohibit direct visual observations.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary form that it is directed to an ion beamimage converter. The image converter has a secondary electron emissionscreen, which is placed in the path of the ion beam to be observed. Anaccelerator screen is positioned to accelerate the secondarily emittedelectrons. Deflecting means is positioned to deflect the electron beamaway from the ion beam path and, finally, a phosphor screen is placedupon the electron beam path so that electrongenerated visible lightcorresponds in shape to the original ion beam so that the ion beam canbe visualized. The preferred deflecting means is electrostaticdeflection.

Accordingly, it is an object of the present invention to provide an ionbeam image converter which makes use of secondary electrons to light aphosphor, and eliminates the possibility of ions striking the phosphor.It is another object to provide an ion beam image converter whichemploys a relatively low field to deflect secondary electrons away fromthe ion beam to a phosphor screen so that only the electron beamimpinges upon the phosphor. It is a further object to provide an ionbeam image converter which operates at a sufficiently low voltage thatthere are no dangerous X-ray emissions during observation. It is afurther object of this invention to provide a visual image of the beamcross section, even at very low ion beam intensities. It is stillanother object to provide a phosphor screen which is bombarded only byelectrons in such an ion beam image converter device so that the life ofthe phosphor screen is essentially unlimited.

Other objects and advantages of this invention will become apparent froma study-of the following portion of the specification, the claims andtheattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view ofthe ion beam image converter of this invention.

FIG. 2 is a section taken generally along the line 2-2 of FIG. I.

DESCRlPTlON Referring to the drawings, the ion beam image converter isgenerally indicated at 10. It is understood that, in use, the ion beamimage converter is positioned in an ion beam device, on the path of anion beam, such an ion beam being indicated at 12. Thus, the ion beamimage converter is located in a housing which is evacuated. The housingis provided with a suitable side porthole for observing the phosphorscreen of the image converter 10, and is equipped with means forproviding electrical connection to the several parts of the ion beamimage converter.

' Ion beam image converter comprises secondary emission screen 14mounted upon screen support 16. The mesh size of this screen determinesthe smallest ion beam that can be observed. Screens having a mesh asfine as 2,000 mesh are available, and these provide a resolutionapproaching 10' meters. The open percentage or transmission of screen 14should be as great as is consistent with the fineness, which isdetermined by the desired resolution. Secondary emission screen 14 ismounted upon its support 16 by means of ring 18. In view of the finenessof screen 14, securing ring 18 is secured thereon to distribute tensionand provide adequate structural support for securement.

Accelerator screen 20 is a large mesh screen; for example, 10 mesh,which is directly supported upon screen support 22; for example, by spotwelding. Screen supports 16 and 22 are secured to each other and arespaced from each other by means of ceramic insulator spacer posts 24 and26. These posts have threaded holes therein, so screws may be engagedthereon, with their heads engaged on the screen supports.

Deflection electrode 28 is the inner deflection electrode for deflectingthe secondary emission electrons out of the ion beam. lnner deflectionelectrode 28 is mounted upon ceramic insulator support post 30, whichpasses through suitable openings in the screen support 22 and is securedupon screen support 16. Deflection electrode 28 is in the form of afractional cylinder having its axis on the desired center of the radiusof curvature of electron deflection.

Outer deflection electrode 32 also has a cylindrical surface. In thiscase, similarly to deflection electrode 28, the electrode is formed aspart of a cylindrical thin metal tube. Opening 34 through electrode 32is positioned on the axis of ion beam l2 so that the ion beam can passtherethrough. Screen 36 covers the opening 34. Screen 36 is a large meshscreen having a large transmission area, at least 90 percent, so thatthe ion beam is substantially unimpeded in its passage. Outer deflectionelectrode 32 is mounted upon ceramic insulator post 38, which passesthrough a hole in support 22, and is mounted on screen support 16. Outerdeflection electrode 32 preferably has as the center of its radius thesame center as the radius of deflection electrode 28.

Phosphor screen 40 is a glass plate with a layer of phosphor powderdeposited thereon. The phosphor screen may be a commercial unit or maybe prepared using conventional techniques of spraying or settlingphosphor powder on a glass plate. The glass plate must first have atransparent conductive coating (i.e., tin oxide) in order to maintainthe electric field and conduct charges away. The type of phosphor powderrequired can be any of the materials normally used for cathode ray tubesor display devices making use of electron beam exitation. Phosphorscreen 40 is mounted by means of support wires 42 and 44 on screensupport 22. Phosphor screen 40 is positioned in such a location as toreceive the beam of deflected electrons, and to be seen through aporthole in the ion beam device. The purpose of the ion beam converteris to provide a nondestructive, indirect means of viewing an ion orelectron beam cross section at any plane in a field-free region alongthe beam axis. It can be utilized to observe, with the naked eye, grossbeam characteristics, such as the size and shape of beam cross section.Since the ion beam image converter is a continuous monitoring device,effects of changes of beam source and optical system parameters on thegross beam characteristics at some plane in the beam system can beobserved. The ion beam image converter is independent of beam energy,and can be applied to beams of very low energy (below 1 k.e.v.) to veryhigh energy (hundreds of m.e.v.).

lons striking secondary emission screen 14 generate secondary electronsin direct proportion to the ion current density. The electrons areaccelerated to the right, as is seen in the drawing, by a uniformelectric field established between screens 14 and 20. Preferably, theion beam would be traveling in a grounded, metal chamber, and screensupport 16 would be mounted in this chamber at ground potential, tomaintain an electric, field-free region up to ion beam image converter10. A positive voltage of between and 20 kv. is applied to theaccelerator screen 20 to accelerate the electrons to provide thenecessary electron energy to illuminate the phosphor in phosphor screen40.

Equal positive and negative deflecting voltages, with respect to thepotential on accelerator screen 20, are applied to the deflectionelectrodes 28 and 32 to maintain a uniform bending of the electrons tothe phosphor with no change in energy. Deflecting voltage should beapproximately 30 percent of the accelerator voltage. For example, ifaccelerator screen 20 has a voltage of kv., the voltages on deflectingelectrodes 28 and 32 would be 13 kv. and 7 kv. respectively. Thephosphor screen 40 is maintained at the same potential as screen 20, bymeans of support wires 42 and 44.

The ion beam image converter 10 can be used for essentially any ion beamenergy. If the ion beam energy is lower than the potential ofaccelerator screen 22, the ions will be decelerated and reversed.Operation in this manner is useful in cases of low-current densitiesbecause additional secondary electrons are generated when the ions arereversed and return to strike screen 14.

For intermediate ion beam energies, comparable to the potential onscreen 20, the ions can be deflected upward and strike deflectionelectrode 32. This will cause no difficulty, because the secondaryelectrons generated by this striking will not have enough energy tolight the phosphor and will not normally follow trajectories allowingthem to reach the phosphor. High-energy ion beams, of sufficient energythat they are unaffected by the deflection voltages, will pass throughthe ion beam image converter to the normal target. Similarly, in thiscase, any secondary electrons generated at deflection electrode screen36, will have insufficient energy to affect the ion beam image on thephosphor.

There is a possibility of X-ray generation at high-beam energies, above100 k.e.v., caused by ions striking the metal screens. To provide properprotection, the deflection plates in this device may be extended,allowing the phosphor to be mounted in a position where lead shieldingof the main chamber will eliminate any hazard.

Since only the electrons are deflected, and the deflected electronslight the phosphor screen 40, the possibility of ions striking thephosphor is eliminated. This is accomplished because the voltages on thephosphor and other electrodes are such as to repel positively chargedions. The energy required for electrons to light the phosphor is in therange of 5 to k.e'.v., causing no X-ray problems during observation. Thephosphor screen is bombarded only with electrons and its life isessentially unlimited. When the energy of the ion beam is sufficientlyhigh to be undeflected by the deflection energy of the ion beam imageconverter, beam monitoring by the image converter 10 can be continuouslycarried out while the ion yerter. The net ion beam transmission throughthe ion beam image converter when there IS no electron beam deflectionapplied depends mainly on the size of the secondary electron emissionscreen 14, which is chosen to provide the desired image resolution. Forexample, an ion beam having a diameter of about 1 mm., can be resolvedwith a screen of mesh (100 lines per inch). The net ion beamtransmission in this case would be about 70 percent. Accordingly, if theion beam image converter 10 is going to be used fairly often, it can beleft in the beam path. On the other hand, if it is only employed duringsetup to focus the beam and optimize the ion source, it can be removedafter these functions have been accomplished.

While electrostatic deflection of the electron beam has been shown inthe drawings, it is clear that electromagnetic deflection canalternatively be employed.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty.

What is claimed is:

1. An ion beam image converter, said ion beam image converter beingpositionable on the axis of an ion beam, said ion beam image convertercomprising:

a screen positionable along the ion beam axis for impingement by the ionbeam so that said screen emits secondary electrons in a patternsubstantially corresponding to the beam impact pattern on said screen;

an accelerator electrode positioned downstream from said screen alongthe ion beam axis, said accelerator electrode being chargeable to apotential to accelerate the secondary electrons along the ion beam axisin the direction of ion flow;

deflection means positioned adjacent said ion beam axis downstream fromsaid accelerator electrode, said deflection means comprising inner andouter electrostatic deflection electrodes respectively positionedinteriorly and exteriorly of a curved electron beam path;

said outer deflection electrode having an opening therethroughpositioned alongsaid ion beam axis, a screen in said opening to permitsubstantial passage of the ion beam along said axis and to maintain auniform electric field across said opening, said inner and outerelectrostatic deflection electrodes being respectively chargeable topotentials higher and lower than said accelerator electrode to causedeflection of the secondary electron beam away from the ion beam axis;and

a phosphor screen positioned on said electron beam path away from saidion beam axis so that electrons striking said phosphor screen representthe ion beam cross section.

2. The ion beam image converter of claim 1 wherein said phosphor screenis electrically connected to be at the same potential as saidaccelerator electrode.

3. The ion beam converter of claim 2 wherein said accelerator electrodecomprises a screen.

1. An ion beam image converter, said ion beam image converter beingpositionable on the axis of an ion beam, said ion beam image convertercomprising: a screen positionable along the ion beam axis forimpingement by the ion beam so that said screen emits secondaryelectrons in a pattern substantially corresponding to the beam impactpattern on said screen; an accelerator electrode positioned downstreamfrom said screen along the ion beam axis, said accelerator electrodebeing chargeable to a potential to accelerate the secondary electronsalong the ion beam axis in the direction of ion flow; deflection meanspositioned adjacent said ion beam axis downstream from said acceleratorelectrode, said deflection means comprising inner and outerelectrostatic deflection electrodes respectively positioned interiorlyand exteriorly of a curved electron beam path; said outer deflectionelectrode having an opening therethrough positioned along said ion beamaxis, a screen in said opening to permit substantial passage of the ionbeam along said axis and to maintain a uniform electric field acrosssaid opening, said inner and outer electrostatic deflection electrodesbeing respectively chargeable to potentials higher and lower than saidaccelerator electrode to cause deflection of the secondary electron beamaWay from the ion beam axis; and a phosphor screen positioned on saidelectron beam path away from said ion beam axis so that electronsstriking said phosphor screen represent the ion beam cross section. 2.The ion beam image converter of claim 1 wherein said phosphor screen iselectrically connected to be at the same potential as said acceleratorelectrode.
 3. The ion beam converter of claim 2 wherein said acceleratorelectrode comprises a screen.